Visible light navigation for C1/C2 fixation and a rare complication related to C2 lamina fracture: case report

Introduction

Os odontoideum is an uncommon craniocervical junction anomaly first described by Giacomini in 1886, with the radiological appearance of an ossicle with smooth cortical margins replacing the C2 dens (1). It is postulated to occur due to congenital failed fusion of odontoid ossification centres, or acquired following post-trauma osteonecrosis after early childhood fracture (2). Regardless of cause, loss of the functional odontoid peg often results in instability (3). This predisposes to the risk of cervical cord compression (1).

Patients with os odontoideum may be asymptomatic, suffer from local symptoms of neck pain and headache, present insidiously with myelopathic symptoms and signs, or suffer from acute catastrophic neurological functional loss (4). Delayed diagnosis is common due to the condition’s rarity (estimated prevalence <1:1,000,000) and overlap in presentation with more common degenerative cervical spinal pathologies. Imaging evaluation is essential for management and includes flexion-extension cervical radiographs to assess for instability, computed tomography (CT) scans for bony morphology and screw planning, and magnetic resonance imaging (MRI) scans for evidence of cord compression.

Symptomatic instability with radiological evidence of cord compression are indications for surgical stabilization. Instability may involve C0–C1 in addition to C1–C2 (5). For C1–C2 instability, posterior C1–C2 fusion, as described by Goel (6) and Harms (7) in the 1990–2000s, may be considered the gold standard for fixation. Traditional C1 lateral mass screw placement requires exposure of the C1–C2 junction and screw entry at the inferomedial aspect of the posterior arch lateral mass junction, risking torrential venous plexus bleeding, and often requires sacrifice of the C2 occipital nerve. In contrast, our radiation free navigated technique utilized a through-the-arch approach to the C1 lateral mass, which carries inherent risks, particularly in patients with anatomical variants. Notably, ponticulus posticus [a bony arch overhanging the vertebral artery (VA) groove] is an anatomical variant that limits the safe window for lateral mass screw insertion which could result in iatrogenic VA injury (8). Our case report describes safe C1 lateral mass screw placement via visible light navigation; however, the patient encountered a C2 lamina screw-related complication causing neurological deficit in the early postoperative period. We present this article in accordance with the CARE reporting checklist (available at https://jss.amegroups.com/article/view/10.21037/jss-25-154/rc).

Case presentation

Clinical history

All procedures performed in this study were in accordance with the ethical standards of the institutional and/or national research committee(s) and with the Declaration of Helsinki and its subsequent amendments. Verbal consent was obtained from the patient for the publication of this case report and accompanying images. A 57-year-old male businessman with good past health was referred to the spine clinic for a 2-year history of insidious onset myelopathy symptoms comprising of reduced bilateral hand dexterity, gait disturbance, tightness from the neck downwards, and loss of temperature sensation over the lower limbs. He was unable to practice his hobby of watchmaking. Physical examination demonstrated that although he could walk unaided, Romberg test for proprioceptive deficit was positive. Four limb motor power was normal, however there was reduced fine touch sensation in the C6–T1 dermatomes of the right upper limb. Hoffman test was positive on the left side, as was a finger escape sign of grade 1. His modified Japanese Orthopaedic Association (mJOA) score was 11.5. Upper limb reflexes were normal, whilst lower limb reflexes were brisk. X-rays demonstrated C1/2 instability secondary to os odontoideum, with radiological changes further defined upon CT (Figure 1). There was evidence of cervical cord tapering and myelomalacia upon MRI images (Figure 2).

Figure 1 Preoperative X-rays and CT scan. (A,B) Static antero-posterior and lateral cervical spine X-rays. (C,D) Dynamic flexion and extension lateral cervical spine X-rays showing evidence of C1/2 instability. (E,F) Os odontoideum on plain CT, mid-sagittal and axial cuts. CT, computed tomography.

Figure 2 Preoperative MRI scan. (A) T2-weighted mid-sagittal cervical MRI and axial cuts progressing from cranial to caudal direction with accordance with cut lines. There is evidence of cord atrophy and myelomalacia. (B-D) Axial T2-weighted cuts proceeding from superior to inferior yellow lines shown upon (A). MRI, magnetic resonance imaging.

Initial surgery

The patient underwent surgery six months after presentation, via C1–C2 fusion assisted by visible light navigation (7D FLASH navigation) (9). This navigation system fuses surface landmarks obtained via a light detection and ranging (LIDAR) camera with preoperative CT fine cut images and does not need intraoperative fluoroscopy. As such, the operating room setup was simplistic without requiring C-arm positioning (Figure 3A), although the wound had to be larger to facilitate camera visualization of C1 and C2 surface landmarks (Figure 3B, approximately 3.5 inches, as the patient had thick posterior spinal musculature). The C1 arch, C2 spinous process and lamina, as well as upper C3 spinous process and lamina was exposed. A navigation tracker was clipped upon the C3 spinous process, followed by registration against a reference CT scan in a process taking less than two minutes. Under navigation (Figure 3C-3F), C1 lateral mass screws were inserted through-the-arch without venous plexus exposure or occipital nerve sacrifice, followed by C2 lamina screws. Total surgical time was 128 minutes, with 50 mL of blood loss. Intraoperative neurophysiological monitoring (somatosensory evoked potentials/motor evoked potentials) was stable. Upon discharge on postoperative day 7, he reported that tightness from the neck and below had subsided, whilst numbness over the right upper limb and lower limbs had decreased. mJOA had improved to 14.5 (recovery rate 55%). Early postoperative radiographs were unremarkable (Figure 4A-4C).

Figure 3 Intraoperative radiation-free navigation. (A) Surgical room setup with registration camera and light source positioned over the wound, and screen placed at caudal end of patient. (B) Tracker, arrowhead-shaped, clipped onto the C3 spinous process, with navigated probe protruding from the cranial wound. (C,D) Right and left C1 probe trajectories during placement of through-the-arch lateral mass screws. (E,F) Right and left C2 probe trajectories during placement of C2 lamina screws.

Figure 4 Postoperative radiological evaluation. Open mouth, lateral, and antero-posterior views of the cervical spine taken at week 1 (A-C) and week 4 (D-F) postoperatively. (G,H) Axial CT scans taken at 5 weeks postoperatively demonstrating satisfactory positioning of C1 lateral mass screws, but bilateral C2 lamina fractures with screws displaced ventrally to within the cervical canal. (I) Preoperative CT scans demonstrating narrow C2 lamina width at 3.2 mm (right) and 3.4 mm (left) over the isthmus. (J,K) Mid-sagittal and axial cut over C2 demonstrating cord impingement from the displaced C2 lamina screw. CT, computed tomography.

Postoperative course

The patient was reviewed in the clinic at 4 weeks postoperatively. He reported a 1-week history of neck pain radiating down to the lateral arm, worse on the right side. Motor power was intact. X-rays were unremarkable (Figure 4D-4F), and he had been compliant on the prescribed Philadelphia neck collar. The patient was prescribed analgesics and an early follow-up for review. Upon review at 5 weeks postoperatively, he reported right sided thigh numbness, right upper limb weakness, and unsteady gait. Right shoulder abduction power had reduced to 2/5, as had distal right upper limb power to 4/5. mJOA had decreased to 11. CT and MRI imaging (Figure 4G-4K) demonstrated bilateral laminar fractures, with both screws displaced into the cervical canal and the right screw causing cord compression.

Revision surgery

The patient received urgent revision surgery after review of CT/MRI. Intraoperatively, bilateral C2 lamina screws were loose whilst the C1 screws were solidly anchored. C2 screws were removed, and instrumented fusion was extended to C3 and C4 via bilateral lateral mass screws (Figure 5), which was not navigated but inserted using the Magerl technique. His bony structures were noted to be inordinately small compared to his body habitus (80 kg). He received 5 weeks of intensive inpatient physiotherapy, upon which there was full return of proximal right upper limb power, with residual weakness (4/5) over the right finger abductors and adductors. He could walk with a stick and standby assistance, and mJOA had improved to 12.

Figure 5 Postoperative X-ray. (A) Lateral and (B) antero-posterior cervical radiographs at 2 weeks post revision surgery, showing extension of fixation to C3 and C4 via lateral mass screws.

Discussion

This case highlights the expedited workflow of visible light navigation-assisted C1–C2 posterior fixation in a case with os odontoideum and myelopathy, as well as a complication of C2 fracture with screw displacement causing cord compression. We hereafter discuss the interplay between anatomy, navigation, biomechanical forces and technical constraints relevant to our case.

To the best of our knowledge, this is the first description on the use of visible light navigation system over the C1/2 levels, as prior literature has only described its application in the subaxial, thoracic, and lumbar spine (9). This facilitated through-the-arch fixation of C1, with avoidance of C1/2 venous plexus exposure and C2 occipital nerve sacrifice, as traditionally described in the Goel-Harms method of fixation. Minimally invasive techniques in combination with visible light navigation are also feasible using a percutaneous spinal module, as has been described for C1/2 fixation using robotic assistance (10). Torrential venous plexus bleeding in particular is a common source of morbidity that may even result in change of operative plan, highlighting a major advantage of through-the-arch techniques (11). As such, blood loss (50 mL) was lower than that of comparable reports of posterior C1/2 fixation with a case series describing an average loss of 200 mL (12). Whilst navigation optimizes static screw trajectory, it cannot detect nor anticipate perioperative events related to hardware such as fracture or loosening. The authors believe that fracture and displacement occurred postoperatively; after burring away the lamina cortex and using a pedicle probe to find the screw tracts, use of a ball-tipped pedicle probe did not identify any obvious cortical breach, and symptom onset was delayed until 4 weeks postoperatively. The 7D system has the advantage of rapid registration between patient landmarks and the pre-op CT without further radiographs for open approaches.

Translaminar screw fixation was first described by Wright in 2004 as an alternate method for C2 fixation (13). The screw trajectory results in a low risk of neurological and vascular injury. Amongst C2 fixation options, cadaveric biomechanical studies have demonstrated lamina screws to have superior pullout strength compared to pars screws, but inferior pullout strength compared to pedicle screws (12). Another cadaveric study similarly indicated the bone/screw interface strength to be strongest in transpedicular fixation, and weakest with C2 pars screws, with intralaminar fixation generating 66% greater peak pull-out force (POF) values than pedicle screws (14). Whilst favoured as a rescue screw, our institution has been using them routinely for C1/2 fixation. A contributing factor to fixation failure in our patient was likely inadequate laminar dimensions, revealed to measure a width of 3.2 and 3.4 mm for the right and left bony corridors, respectively (Figure 4H,4I), compared to the screw size of 3.5 mm. Amongst cases with os odontoideum, a radiographic study described C2 lamina width and length to be 6.95±0.82 (5.5–8.8 mm) and 25.60±2.18 mm, respectively (15). Another radiological study described that 93% of specimens had a laminar thickness ≥4 mm (16). Lamina width in our patient was inordinately small, the lesson to learn being that the laminar corridor should be considered in all cases of C1/2 fixation during meticulous pre-op planning. Apart from screw tract dimensions, other aspects for assessment comprises of the VA course and dominance, especially as vascular anomalies have been described in conjunction with os odontoideum (17), as well as C1/2 reducibility for potential transarticular fixation (18). Cortical breach has been investigated following lamina screw insertion (19) but with reported rates lower than that of C2 pedicle screws (1.3% vs. 7%) (20), whilst our case further illustrated the sequelae of screw cutout causing cord compression. Strengths in this case report include detailed intra- and perioperative photos/images as well as serial documentation of neurological findings. However, the diagnosis of lamina screw cutout was based on educated deduction and intraoperative placement error remains a possibility.

Conclusions

Visible light navigation may be used for the cervical spine. Surgeons should be familiar with how to evaluate for fixation modalities around C1/C2, as well as with their pitfalls and potential complications.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the CARE reporting checklist. Available at https://jss.amegroups.com/article/view/10.21037/jss-25-154/rc

Peer Review File: Available at https://jss.amegroups.com/article/view/10.21037/jss-25-154/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jss.amegroups.com/article/view/10.21037/jss-25-154/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All procedures performed in this study were in accordance with the ethical standards of the institutional and/or national research committee(s) and with the Declaration of Helsinki and its subsequent amendments. Verbal consent was obtained from the patient for the publication of this case report and accompanying images.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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